Solar power is one of the more desirable types of renewable energy. For years it has been touted as one of the most promising for our increasingly industrialized society. Even though the amount of solar power theoretically available far exceeds most, if not all, other energy sources (renewable or not), there remain practical challenges to utilizing this energy. In general, solar power remains subject to a number of limitations that have kept it from fulfilling the promise it holds. In one regard, it has been a challenge to implement in a manner that provides adequate electrical output as compared to its cost. The present invention addresses an important aspect of this in a manner that significantly increases the ability to cost-effectively permit solar power to be electrically harnessed so that it may be a cost-effective source of electrical power.
One of the most efficient ways to convert solar power into electrical energy is through the use of solar cells. These devices create a photovoltaic DC current through the photovoltaic effect. Often these solar cells are linked together electrically to make a combination of cells into a solar panel or a PV (photovoltaic) panel. PV panels are often connected in series to provide high voltage at a reasonable current. This may be accomplished to make electrical interconnect losses low. The output of a solar cell or a solar panel, or even combinations thereof, is frequently then converted to make the electrical power most usable since the power converters often employed can use high voltage input more effectively. Conventional power converters sometimes even have at their input handled by an MPPT (maximum power point tracking) circuit to extract the maximum amount of power from one or more or even a string of series connected panels. One problem that arises with this approach, though, is that often the PV panels act as current sources and when combined in a series string, the lowest power panel can limit the current through every other panel.
Furthermore, solar cells historically have been made from semiconductors such as silicon pn junctions. These junctions or diodes convert sunlight into electrical power. These diodes can have a characteristically low voltage output, often on the order of 0.6 volts. Such cells may behave like current sources in parallel with a forward diode. The output current from such a cell may be a function of many construction factors and, is often directly proportional to the amount of sunlight.
The low voltage of such a solar cell can be difficult to convert to power suitable for supplying power to an electric power grid. Often, many diodes are connected in series on a photovoltaic panel. For example, a possible configuration could have 36 diodes or panels connected in series to make 21.6 volts. With the shunt diode and interconnect losses in practice such panels might only generate 15 volts at their maximum power point (MPP). For some larger systems having many such panels, even 15 volts may be too low to deliver over a wire without substantial losses. In addition, typical systems today may combine many panels in series to provide voltages in the 100's of volts in order to minimize the conduction loss between the PV panels and a power converter.
Electrically, however, there can be challenges to finding the right input impedance for a converter to extract the maximum power from such a string of PV panels. The aspect of extracting power at a maximum power point is often referred to as MPP tracking. Some such systems exist, however, there remain limitations, some of which are discussed here. First, the PV panels may act as current sources. As such, the panel producing the lowest current may limit the current through the whole string. In an undesirable case, if one weak panel is producing moderately less, it might become back biased by the remainder of the panels. Reverse diodes can be placed across each panel to limit the power loss in this case and to protect the panel from reverse breakdown.
In systems, at least the following problems can arise and cause some degree of loss in solar energy harvesting:                A. Non-uniformity between panels.        B. Partial shade        C. Dirt or accumulated matter blocking sunlight        D. Damage to a panel        E. Non-uniform degradation of panels over time        
It may also be troublesome when expensive PV panels are placed in series and the weakest panel limits the power from every other panel. Unfortunately, the series connection may be desired to get high enough voltage to efficiently transmit power through a local distribution to a load, perhaps such as a grid-tied inverter. Further, in many systems, the PV panels may be located on a rooftop, such as for a residential installation. And the inverter is often located at a distance from the rooftop, such as by the power meter or the like. So in embodiments, a way to connect the panels in series but not suffer the losses caused by the lowest power panel, or any series parallel combination, may be needed. There may also be a desire to use unlike types of panels at the same time perhaps without regarding to the connection configuration desired (series or parallel, etc.).
The techniques of photovoltaic power conversion have been recognized as an important limit to solar energy ultimately realizing its potential. Methods of solar power conversion have been proposed that utilize DC/DC converters on each panel along with an MPP circuit as one attempt to enhance the efficiency of energy harvesting when utilizing strings of solar panels. Such attempts, however, have resulted in unacceptably low efficiencies that have made such approaches impractical. These techniques have even been dismissed to some degree by those considering such issues. For example, in the article by G. R. Walker, J. Xue and P. Sernia entitled “PV String Per-Module Maximum Power Point Enabling Converters” those authors may have even suggested that efficiency losses were inevitable but that this module approach held advantages, even though it was attended by poor efficiency. Similarly, two of the same authors, G. R. Walker and P. Sernia in the article entitled “Cascaded DC-DC Converter Connection of Photovoltaic Modules” suggested that the needed technologies are always at an efficiency disadvantage. These references even include an efficiency vs. power graph showing a full power efficiency of approximately 91%. With the high cost of PV panels operation through a low efficiency converter is simply not acceptable in the marketplace.
Another less understood problem with large series strings of PV panels may be with highly varying output voltage, the inverter stage driving the grid my need to operate over a very wide range also lowering its efficiency. It may also be a problem if during periods of time when the inverter section is not powering the grid that the input voltage to this stage may increase above regulatory limits. Or conversely, if the voltage during this time is not over a regulatory limit then the final operational voltage may be much lower than the ideal point of efficiency for the inverter.
In addition, there may be start-up and protection issues which add significant cost to the overall power conversion process. Other less obvious issues affecting Balance of System (BOS) costs for a solar power installation are also involved. Thus, what at least one aspect of electrical solar power needs is an improvement in efficiency in the conversion stage of the electrical system. The present invention provides this needed improvement.